1. Field of the Invention
The present invention relates to light-emitting devices. More particularly, the present invention pertains to a light-emitting device comprising an element (hereinafter referred to as xe2x80x9cEL elementxe2x80x9d) in which a thin film of a light-emitting organic material (hereinafter referred to as xe2x80x9corganic EL filmxe2x80x9d) that can attain electro luminescence (hereinafter also referred to as xe2x80x9cELxe2x80x9d) is interposed between an anode and a cathode. Examples of such light-emitting devices according to the present invention include an organic EL display and an organic light-emitting diode (also referred to as xe2x80x9cOLEDxe2x80x9d in short).
Light-emitting materials which can be used in the present invention encompass all light-emitting materials that can emit light (phosphorescence and/or fluorescence) via at least one of a singlet excitement and a triplet excitement.
2. Description of the Related Art
Since Eastman Kodak Co. announced that a light-emitting device using an organic EL film had emitted light at a low drive voltage, the light-emitting device using the organic EL film has been noticed. According to an announcement by Eastman Kodak Co., it is characteristic to decrease the drive voltage by taking a laminate element structure; therefore, many companies have conducted research and development on such laminate element structure.
Though the organic EL film is individually used as a luminescent layer in some cases, it, ordinarily, forms an EL element in combination with an organic material such as a hole injection layer, a hole transport layer, an electron transport layer or an electron injection layer. On this occasion, the organic EL film includes a polymer-typed organic EL film and a low molecular weight-typed (monomer-typed) organic EL film.
Generally, in the present specification, polymer-typed organic materials are referred to as polymer films whereas low molecular weight-typed organic materials as low molecular weight films so that polymer-typed organic EL films are the polymer films whereas low molecular weight-typed organic EL films are the low molecular weight films.
Now, a structure of the EL element, on which the present inventors has run a test, is shown in FIG. 2. In FIG. 2, an anode 202 comprising an indium tin oxide (ITO) film is provided on a substrate 201; on the thus provided anode 202, provided are a hole injection layer 203 comprising polythiophene (PEDOT), a luminiscent layer 204 comprising poly(p-phenylenevinylene) (PPV) and a cathode 205 comprising a metallic film in sequence in a direction of departing from the anode 202.
As a result of a careful observation by the present inventors on the EL element of a trial production with the structure shown in FIG. 2, many pinholes and delaminations were found in the cathode 205. Further, it was found that such pinholes and the like had caused a dark spot (deteriorated portion in black dot form) of the EL element. That is, the dark spot derived from a defect of film formation of the cathode 205 has remarkably deteriorated a displayed image quality.
The present inventors have considered that such pinholes or delaminations of the cathode 205 are caused by a poor adhesion between the luminiscent layer 204 comprising the polymer film and the cathode 205 comprising the metallic film. That is, the present inventors have considered that, since the polymer film such as PPV is hydrophobic, it shows a poor adhesion with the metallic film thereby causing a pinhole or delamination at the time of the film formation of the cathode 205.
Under such circumstances, it is an object of the present invention to provide a technique for forming a film of a cathode comprising a metallic film with a good adhesion in an EL element with a polymer film as a light-emitting layer or an electron transport layer. It is another object of the present invention to provide a light-emitting device with a favorable displayed image quality to be produced by utilizing the technique.
The present inventors have considered that the adhesion can be improved by not performing a film formation of the metallic film directly on a polymer film but by providing a low molecular weight film on the polymer film as a buffer for the purpose of improving the adhesion and then providing the metallic film on the thus provided low molecular weight film.
That is, when a luminescent layer is the polymer film, the low molecular weight film is provided as an electron transport layer or an electron injection layer and then the cathode comprising the metallic film may be provided on the thus provided low molecular weight film. Further, when the electron transport layer is the polymer layer, the low molecular weight layer is provided as the electron injection layer and then the cathode comprising the metallic film may be provided on the thus provided low molecular weight film.